the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Oct 2023
The characteristics of tides and their effects on the general circulation of the Mediterranean Sea
Abstract.
The effects of tides on the Mediterranean Sea’s general circulation, with a particular focus on the horizontal and vertical currents, are investigated using twin simulations with and without tides. Amplitudes of tides in the region are typically low, but an analysis of the kinetic energy demonstrates that tides have effects across many spatial and temporal scales in the basin, including nonlinear effects at short periods (less than one day) with high kinetic energy peaks at near-inertial, basin modes and tidal frequencies. Internal tidal waves are also revealed below 100 m. Tides are found to amplify several basin modes of the Mediterranean Sea, broaden several tidal frequency energy spectra bands, as well as interact energetically with near-inertial waves. Tides are found to increase the mixed layer depth in the Mediterranean Sea, particularly in the deep and intermediate water formation areas of the Western and Eastern basins. It is confirmed that the net mass transport through the Gibraltar Strait is unchanged by tides, but both inflow and outflow increase.
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RC1: 'Comment on egusphere-2023-2251', Anonymous Referee #1, 26 Nov 2023
The manuscript "The characteristics of tides and their effects on the general circulation of the Mediterranean Sea" by McDonagh et al. presents a study investigating the effect of tides on the Mediterranean Sea circulation. To my knowledge, this subject has not yet been extensively investigated. Apart from Sannino et al. 2014, which provided a first analysis of the tidal influence on the large-scale Mediterranean circulation, previous studies have only focused on specific areas such as the Alboran Sea (Sanchez-Garrido et al., 2013) or the Sicily Strait (Oddo et al., 2023; Gasparini et al., 2004). As such, the present manuscript proposes a valuable contribution to reinforce and deepen the current knowledge on the tidal influence on the Mediterranean Sea.
In this study, the authors diagnose the effect of tides from a pair of 5-year-long tidal and non-tidal experiments based on very similar numerical configurations. The first and second sections of the manuscript investigate the influence of tides on the Mediterranean Sea dynamics through the prism of sea level and kinetic energy spectra. These analyses focus on three specific locations: the Strait of Gibraltar, the Tyrrhenian Sea, and the Cretan Sea. They reveal that tides impact high-frequency (> 1 day⁻¹) dynamics through the propagation of the main tidal harmonics at work within the Mediterranean Basin and their interaction with various basin-scale modes. Then, the authors relate the tidally enhanced dynamics and mixing to the deepening of the mixed layer depth in the tidal simulation. The two final sections discuss the impact of tides on the thermohaline properties of the Mediterranean Sea and the transports through the Strait of Gibraltar.
Overall, the manuscript has the potential to provide valuable results on tidal contribution to Mediterranean dynamics. However, further work must be done before it can be accepted. More specifically, I think that although the two first sections of the manuscript provide valuable results, they focus on too specific areas to provide an overall picture of the tidal influence on the high-frequency Mediterranean dynamics. In addition, to assess the influence of tides on the "general circulation of the Mediterranean Sea", as stated in the the title of the study, the manuscript should investigate the influence of tides on the long-term, large-scale circulation. Regarding the impact of tidal dynamics, section 5 should more clearly distinguish the influence of local tidal mixing at the strait of Gibraltar, which impacts the Mediterranean mixed layer depth indirectly by changing the density of Atlantic water masses, and the less intense mixing induced by tidal currents throughout the Mediterranean Sea. The former is not directly related to the interaction of tides with the Mediterranean circulation, and it has already been investigated with similar model configurations (Sannino et al., 2014; Naranjo et al., 2014). Thus, I suggest not including it in this manuscript. On the other hand, the mixing induced by tidal dynamics is relevant to this study. Finally, sections 6 and 7 are, in my opinion, outside of the scope of this paper. Although interesting, the corpus of these sections has no apparent link with Mediterranean circulation and mainly emphasizes the conclusions of previous studies without additional results.
For these reasons, I am arguing for a major revision of the manuscript. Specifically, I would suggest:
- In sections 3 and 4: Add 2D maps of the tidal influence over the specific frequency bands mentioned in the text to give further confidence in the spatial extension of the results discussed.
- In section 5: If you intend to show that the tide-enhanced high-frequency dynamics are responsible for the deepening of the Mediterranean mixed layer depth, you should:
(1) Mostly focus on the overall increase of the mixed layer depth rather than its local increase over deep convection areas, where the intensity of tidal mixing is unlikely strong enough to drive the deepening.
(2) Look at the seasonal cycle of the mixed layer depth and stratification, as it would be easier to separate the effect of local vertical mixing from that of the tide-induced densification of Atlantic water masses at the Strait of Gibraltar. - Add some results on the influence of tides on the Mediterranean large-scale circulation, or reformulate the title of the article only to consider the high-frequency dynamics.
- Put sections 6 and 7 in the supplementary materials or a "model validation" section, demonstrating the consistency of the model with previous studies.
I do encourage authors to make the necessary effort to improve this manuscript. You can find general and detailed remarks, as well as references, in the attached PDF.
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AC1: 'Reply on RC1', Bethany McDonagh, 18 Mar 2024
We appreciate the time and effort taken to write this review, which has provided us with very useful feedback for this paper. We have considered and addressed all comments and questions from the reviewer and believe that we have improved the manuscript overall. Attached is the full response to the comments.
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RC2: 'Comment on egusphere-2023-2251', Anonymous Referee #2, 08 Dec 2023
This paper undertakes an assessment on a basin scale of the effects of tides on the Mediterranean Sea’s circulation. The methodology involves an analysis of two experiments, one with tides and one without. It should be noted that tides aside, the experiments do have some other differences. To my knowledge there has not been such a systematic analysis of the effect of tides to the Mediterranean basin as a whole and expands on several studies which focus on specific sub regions. Thus, the subject matter of this paper is both very relevant to the journal and makes a substantial contribution beyond what has been done to date.
The twin experiments are of 5 years duration. To understand the effect on the Mediterranean dynamics the first parts of the paper focus on sea level and kinetic spectra in the following locations, the Strait of Gibraltar, the Tyrrhenian Sea, and the Cretan Sea. Clear Amplification of several sub daily modes is demonstrated by the analysis. The next section goes on to investigate how this tidal amplification leads to deepening of the mixed layer depth. To finish the paper discusses the effects of the tides on the thermohaline structure of the Mediterranean and briefly the baroclinic transport through the Strait of Gibraltar.
There are a number of points of improvement that I have suggested in more detail in the attached pdf.
In brief:
I am not so sure section 7 adds to the paper in its current form. It might be better to omit it, as it's probably beyond the scope of this paper to go into a thorough tidal excursion analysis here.
The twin experiments are not quite identical other than the tides and a short simulation could shore up the inference that the other changes are trivial compared to the tides themselves.
Overall in my opinion both the subject matter and the analysis done in this paper are suitable for publication, but some further changes as suggested in the pdf are required to bring it to publication standard and enhance the work already done to date by the authors.
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AC2: 'Reply on RC2', Bethany McDonagh, 18 Mar 2024
We appreciate the efforts made for this very useful review of our manuscript. In the attached file, we have addressed each comment made and detailed the changes to the manuscript.
Citation: https://doi.org/10.5194/egusphere-2023-2251-AC2
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AC2: 'Reply on RC2', Bethany McDonagh, 18 Mar 2024
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EC1: 'Comment on egusphere-2023-2251', Joanne Williams, 24 Mar 2024
Dear authors,
thank-you for your responses to the reviewers and for the substantial changes you have made to the manuscript. I think it is much improved and you have addressed all the key points raised, but given the scale of the changes I have asked reviewer 1 to take a second look.
In the meantime there's some other points to tidy up, you can wait till the review comes in but please do address these at that point:
You will need to improve several of the figures. Nearly all of the figure fonts are far too small to be legible, they should be no smaller than a size or two down from the main text, so it's not necessary to zoom in too much. Please refer to the author guidelines for the precise requirements. In some cases you might simply be able to make the figure the full width of the page, but consider each in turn, eg the scale on fig 7&8 would be more legible if you change the units to mm/s and avoid the exponent.
Fig 1 - the place labels are illegible. I think this is a matlab plot - try looking for the "color" or "backgroundcolor" property of the text labels (I can't remember which) and set to a color. Thanks for providing the map though.
In Fig 10 right hand, it would be more intuitive to plot the mixed layer depth on a reversed axis (set(gca,'ydir','rev') in matlab) so lower down the page is a deeper MLD.
You will also need to make the data available, please link to a suitable repository.
Best wishes,
Joanne Williams, editor.Citation: https://doi.org/10.5194/egusphere-2023-2251-EC1 -
AC3: 'Reply on EC1', Bethany McDonagh, 26 Mar 2024
Dear Joanne,
Thank you for these comments. We went back through the figures and increased the font sizes on many of them, and made the other edits that you suggested. All of the text on the figures is now clearly visible and should be legible on any device, and we will upload this version after hearing from the reviewer, in case further changes are needed.
We look forward to hearing from reviewer 1, and will upload our data to a repository in the meantime.
All the best,
Beth McDonagh, on behalf of the authors
Citation: https://doi.org/10.5194/egusphere-2023-2251-AC3
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AC3: 'Reply on EC1', Bethany McDonagh, 26 Mar 2024
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